Testing resistor

[flamtech]

Hi

I am new to electronics. I know which end of the soldering iron gets hot!

I am in the southern hemisphere which means winter is upon us.

I want to get my electric blanket working again as I hate to get into a cold bed.

When I turn the blanket on, the indicator light turns on, but the blanket doesn't get warm.

All the components test for continuity if I use my multimeter accept the resistor situated at "R3" and the indicator bulb on the board. Apparently that is a 120k resistor?

I went to a shop and bought 4 resistors with the Brown, Red, Yellow and Gold bands.

Before trying to replace the resistor, I wanted to test the resistor using my multimeter and I could not get a reading on my multimeter.

So now I am concerned that I am not testing the resistor in the correct manner, and maybe the problem isn't with the said resistor.

The electronic shops doesn't want to do the repair, so if I mess up the switch it will be no big problem, because its worthless in any case, but maybe I can use it to learn something about circuit boards.

Thanks in advance.

 

[Harald Kapp]

As far as I can see the resistor is only a current limiter for the indicator light. Since the light goes on, the resistor should be o.k..

Have you measured the resistance of the heating elements in the blanket?

I assume the copper contacts are part of the switch and the switch has 2 or three positions four different heat settings.
Can you supply a schematic on how the contacts of the switch are operated (which contacts are closed, which ones open) in the different heat settings?
Maybe you could even disentagngle the PCB layout and produce a schematic diagram?

Harald

BTW: temperatures going up to over 20 °C here

 

[Rleo6965]

36.5 °C here

 

[flamtech]

The switch has 4 settings Off, 1, 2, and 3

The reading between N1 (Brown) and N2 (Blue) is .438

The reading between W1 (Black) and W2 (White) is .556

Inside switch

 

[shrtrnd]

If you've checked your fuse, I'm with Harald, you've probably got a break in your
blanket heating elements, or the wires connecting to them.

 

[KJ6EAD]

You can test the diodes with your meter and continuity through the fuse and what looks like some kind of thermal switch positioned between a pair of 2k resistors acting as heaters.

Another possibility to check is that the switch is making good contact since this is partially dependent on the case halves being tightly bonded.

 

[Rleo6965]

Have you tried directly test resistance of 4 wires.? not including diodes .This could be 2 heating element.

 

[(*steve*)]

The easiest thing to do is to measure the resistance across the mains plug.

Change from one setting to another and see if you get anything less than approximately infinite resistance.

My guess is that you wont. It indicates that you have breaks in enough elements that continuity does not remain on any setting.

It's possible that you could find some element that has conductivity and use just that, but it may get far too hot (you don't want t start a fire).

Alternatively it could be switch contacts,or a fuse somewhere. But the contacts look clean, and a fuse failing seems unlikely without some other failure that seems unlikely for an electric blanket..

 

[flamtech]

Have you tried directly test resistance of 4 wires.? not including diodes .This could be 2 heating element.

The reading between N1 (Brown) (Bottom in picture) and N2 (Blue) (Second from Bottom) is .438

The reading between W1 (Black) (Second from top) and W2 (White) (Top) is .556

 

[Rleo6965]

How about placing jumper wire as temporary switch contact. This is to rule out defective switch.

 

[Harald Kapp]

I'm with the others: If the resistors and diodes on the PCB measure o.k., then it is most likely the blanket's heating elements or one the wires that go to them that are broken.

Sorry,

Harald

 

[flamtech]

I replaced the resistor, and my blanket is working.

What I still want to know is; can I test the resistor with my multimeter, and how do I do it.

 

[KJ6EAD]

Just turn the knob to the 200k scale and measure it. It's a little surprising that the resistor was open. Please post your results.

 

[(*steve*)]

put the multimeter in a resistance mode and connect the probes to the resistor. Then read the value.

Do this with the device you've removed from the circuit.

 

[flamtech]

Just turn the knob to the 200k scale and measure it. It's a little surprising that the resistor was open. Please post your results.

Thank you. The one that I removed tested at 117.8. So it was not the resistor that was faulty.

Thank you to all the people that replied to my questions, I appreciate it.

 

[(*steve*)]

If replacing the resistor fixed the problem, the problem may have been a bad solder joint.

It might be worthwhile looking over the board and touching any which appear suspicious with your iron.

 

[KJ6EAD]

If replacing the resistor fixed the problem, the problem may have been a bad solder joint.

That's what I was thinking. Besides seeing the trace pattern, this is another reason to show both sides of a PCB when posting. It's so hard to get children, slaves and prisoners in the third world to produce consistently good solder joints.